CA9 and breast cancer: Interestingly, MCT1 and MCT4 have also been reported to form a metabolon with the transmembrane carbonic anhydrase IX (CAIX; another HIF-1 target gene [120] and a pH regulator catalyzing the reversible hydration of CO2 to bicarbonate and proton [121]) in breast cancer cells (and not healthy tissue) through its binding to the CD147 chaperone [122].